The present invention relates to a semiconductor nonvolatile memory of the floating gate type used in electronic instruments, such as computers, and which is erasable by irradiation with ultraviolet rays.
Conventionally, as shown in FIG. 2, a semiconductor nonvolatile memory cell of the floating gate type erasable by ultraviolet rays is constructed such that a source region 2 of N.sup.+ type and a drain region 3 of N.sup.+ type are formed in a surface of a P type semiconductor substrate 1, and a floating gate electrode 5 is formed over a channel region between the source region 2 and the drain region 3 on the semiconductor substrate surface, the floating gate electrode 5 being covered with an electrically insulating film. This type of memory cell is disclosed, for example, in M. Wada et al. "Limiting Factors for Programming EPROM of Reduced Dimensions" in International Electron Devices Meeting (IEDM) Dig. Tech. Papers, pp 38-41 (1980).
However, in the conventional semiconductor nonvolatile memory cell, as shown in FIG. 2, the substrate 1 has formed in the surface thereof an impurity-doped region 8 having an impurity density higher than that of the substrate 1 so as to reduce program time and to separate or isolate memory cells from each other. Therefore, the memory has a high threshold voltage of about 1.5 V after the erasing by ultraviolet light and therefore the memory has a drawback that the operation is difficult in the low voltage range.